I am currently working in singapore on a work permit. A flat 15% of my income is deducted as tax. I was inform

No deduction of tax should be allowed unless you have a arrangement with the tax authority to deduct your tax.

If you work for more than 186 days with annual income less than $20,000 (net of relief), then you do not pay tax.

So your employer should not deduct 15% of your income as tax. Your employer normally will withhold your last month salary if you resign. In Singapore, employer is not accountable for your income tax as long as the employer duly inform the tax authority when you resign and withhold the salary for last month as indicated.